Method for galvanizing steel



June 22, 1965 w. H. WRIGHT 3,190,763

METHOD FOR GALVANIZING STEEL Filed July 24. 1961 FIG I L M A Thin layer of Iron Zinc L F mm.

FIG 2 e lronlinc Alloy L w 1 Fe FIG 3 Alloys 56 INVENTOR. F I G 4 WILBERT H. WRIGHT Slaw W A "URNE Y5.

United States Patent Olfice 3,190,768 Patented June 22, 1965 3,190,768 METHODFGR GALVANIZING STEEL Wilbert H. Wright, Weirton, W. Va., assignor to National Steel Corporation, a corporation of Delaware Filed .luly 24, 1961, Ser. No. 126,327 1 Claim. (Cl. 117-932) The invention is concerned with novel methods for dull-coat galvanizing flat-rolled steel product having a tight-coat, dull-tone finish.

Galvanized coatings ordinarily have a large crystalline structure presenting a frost-flower or spangled surface. There are many apt uses for galvanized product where the spangled surface is not desirable. The long association 'of the spangled surface with trash cans, and the like, make it unsuitable for decorative purposes. It is also difficult to'coat with paint or plastic; neitherwill spread readily without pretreatment of the coating. And, in spite of a uniform coating thickness, large spangles present an uneven surface at the crystal boundaries which is highlighted by paint or plastic.

Proper dull-coat galvanizing can overcome these and other drawbacks of conventional galvanized product. The main objective of this invention is production of tightcoat galvanized flat rolled product which: has a matte finish; is smooth and uniform in appearance, and to the 'touch; and has excellent paint, plastic, or similar coating,

adherence qualities. An important contribution of the invention is its ready application to continuous strip galvanizing operations.

Attempts have been made in the prior art to produce a dull finish on galvanized product. Minimized spangle is disclosed by Hill et al., US. Patent No. 2,169,864, and Cook et al., US. Patent No. 2,094,583. Galvannealing is disclosed in Making, Shaping and Treating of Steel,

Sixth Edition, Special Finishing Operations, Section XIII,

pp. 958 and 959. These and similar teachings expose several limitations of the prior art. For example, they are directed to treating sheets rather than continuous strip and, whether applying steam, a chemical solution, or heat, all concentrate their treatment on the galvanized coating. Where heat is employed in the prior art, the teachings have been limited to minimum coating thicknesses much below the so-called Commercial Coatings of ordinary galvanized product. This limitation was deemed necessary in order to maintain good coating adherence and, possibly, to limit running of the coating during treatment and brittleness after treatment. The invention employs heat but is not so limited; its teachings permit production of spangle-free, dull-coat galvanized product with any of the standard coating thicknesses and without damage to the coating, its adherence qualities, or

' its ductility.

In describing the invention reference will be had to the accompanying drawings in which:

FIGURE 1 is a schematic illustration of a continuous strip galvanizing line embodying the invention; and FIGURES 2, 3 and 4 illustrate photomicrographs of 1 practice, furnace may be a multiple pass-continuous annealing furnace or merely a heating furnace which brings the strip to the proper temperature for galvanizing. The strip is then led: through atmosphere controlled chutes 22 and 24 into galvanizing pot 26; through molten. .70

bath 28 around submerged roll 30, and through coating concentration of heat in the base metal.

strip galvanizing line is deemed necessary for an understanding of the invention and it is understood that the continuous galvanizing practice employed places no limitation on the invention.

To produce the desired dull-coat finish, the invention alloys galvanize coating with the steel base metal by induction heating of the galvanized strip. Coated strip 32, after exit from the coating rolls 34 passes through induction heating unit 36 which is magnetically linked with the coated strip 32 during such passage. Energy from induction unit 36 is transferred by electromagnetic induction to the steel strip where it is converted into heat. Heat is transferred to the galvanized coating, raising the temperature of the coating above its melting point and causing substantially complete alloying of coating metal with the steel strip.

The photomicrographs of FIGURES 2, 3, and 4 show untreated, partially treated, and treated strip. The untreated galvanized strip 40 of FIGURE 2 includes steel strip 41, galvanize coating 44, and a boundary layer 45 between the strip and the coating. The boundary layer may contain traces of iron-zinc alloy.

FIGURE 3 illustrates partially dulled strip 47 which includes steel strip 48, a layer 'of alloy 49, and pure galvanize coating 50. About A of the coating is alloy; the strip is no longer completely bright, since large angular alloy crystals 51, 52, 53 penetrate the outer coating surface; nor is it uniformly dull.

Coated strip 55 of FIGURE 4 includes steel strip 56 and alloy 57. The original galvanize coating has been converted substantially entirely to identical columnar crystals of iron-galvanize alloy and a uniform, matte, dull-tone finish produced.

An important advantage of induction heating is the When it is attempted to introduce heat to the base metal by radiation, convection, or conduction, the heat must be forced through the coating. This can result in damage to the coating or runningof the coating before proper alloying takes- 'the coated strip is heated from the inside out.

It has been found that by raising the coated strip to the proper temperature the strip will go dull without the necessity of maintaining the strip in a heated condition; also, that raising the strip to the proper temperature quickly is a major factor in producing good coating adherence and ductility. Continuous strip operations and induction heating combine ideally to take advantage of these teachings of the invention.

In continuous strip operations the strip should reach the proper temperature within the short distance spanned by the induction unit; in practice units of approximately 24" and 38" have been employed. Coloration of the strip is one method of determining the proper temperature.

It has been observed that the strip should be raised, as a minimum, to a temperature where red coloration starts to appear in order to produce the desired dull-coat in continuous strip operations.- In appearance this color is a very dark cherry-indicating a temperature of about 1050" F. or slightly higher. The temperature cannot be as accurately pinpointed as with a color chart for uncoated steel because the effect of the galvanize coating on the observable color is not certain. However good dullcoat was produced when colors from dark cherry through control rolls 34. No further explanation of a continuouscherry red were observed. Using the color chart provided by Tempil Corporation 132 W. 22nd Street, New York 11, New York, with their Basic Guide to Ferrous Metallurgy, copyrighted 1954, this would indicate a broad a) range of temperatures, between approximately 1050 F. and 1400 F. although it is believed the optimum range is between 1100 F. and 1200 F.

Aluminum additions to a galvanize coating bath are common and may vary from 0.01% to 1% or higher; see Use of Aluminum in Hot-Dip Galvanizing, by M. L. Hughes, Journal of the Iron and Steel Institute, September 1950, pp. 77-84. It has been found that aluminum plays an important role in dull-coat galvanizing. Its importance lies chiefly in its effect on the percentage of iron in the dull-coat alloy. At any given temperature, the lower the percentage of aluminum in the galvanize coating being treated, the higher the percentage of iron in the alloyed coating. Of course the galvanize coating being predominantly zinc and the base metal being predominantly iron, the dull-coat alloy is predominatly iron-zinc alloy. This alloy must extend to the outer surface of the coating to produce good dull-coat and aluminium can retard this alloying. The action of aluminum in retarding alloying will be discussed in more detail later.

The percentage of iron in dull-coat alloy has an important effect on coating properties such as adhesion, ductility, durability and even color. In general, the higher the percentage of iron in the coating, the lower the ductility and adhesion of the coating. Good tight coat can be maintained however with percentages of iron up to 20% or slightly higher.

Regarding durability, iron can greatly extend the life of hot-dip galvanize coating. At percentages near and above the resistance to atmospheric corrosion is greater than that of the regular galvanize coating. This resistance to corrosion increases with increasing percentages of iron up to slightly above In accordance with the teachings of the invention, dull-coat galvanize .can be produced having the desired uniform appearance as well as a percentage composition of iron between about 10% and 20%.

The percentage aluminum and the dull-coating temperature employed affect the color of the dull-coat. At lower temperatures the dull-coat will have a silver color. As the coating is alloyed at higher temperatures it will be either silvery-white, gray, gun-metal, or yellowish as the temperature increases. If the percentage of aluminum is held down in the molten bath and the applied galvanize coating, the whitish caste will be diminished; below about 0.10% aluminum the white will normally not appear, above about 0.18% aluminum the white will usually persist through a larger portion of this color temperature relationship.

A part of the invention is its teachings on the application of induction heating to continuous operations. With induction heating, galvanized strip can be raised quickly and precisely to the desired alloying tempera-ture and economically acceptable line speeds can be maintained.

Cir

Preferably the strip is treated as it leaves the coating pot while the coating is still molten in order to take advantage of the heat in the strip; also the pot temperature may be raised during dull-coating operations to increase the heat in the strip.

The line speed permissible in continuous-strip operations is dependent on many factors such as physical location and dimensions of the induction unit, the power generated in the strip, the gauge of the steel, the coating thickness, the temperature of the strip as it enters the induction unit, etc. Ordinarily the greater the coating thickness the higher the strip temperature employed in continuous operations. The lighter gauges of steel ordinarily require a higher temperature because of the rapidity with which they give up their heat. The proper frequency for the induction unit is dependent in part on the power required to be generated in the strip; in general, more power can be generated at higher frequencies. The proper frequency is also dependent in part on the thickness of the strip. The lighter the gauge of steel strip the higher the frequency should be since the frequency affects the depth of penetration of the current induced in the strip. The lower the frequency the more the penetration. Ideally the penetration should be no more than approximately one-third of the strip thickness from each side. Beyond this penetration, the currents on the two sides of the strip begin to nullify or counteract. The strip can still be heated but not as efficiently. In the dull-coating practice discussed later, a frequency of 9600 cycles per second was employed largely because this frequency is readily obtainable using an inductor alternator and a capacitive reactance circuit. The power generated in the strip at 9600 c.p.s. was sufiicient to dullcoat at the line speeds employed but more efficient heating is obtainable with higher frequencies. Below the Curie temperature for steel (about 1425 F.) the depth of penetration at 9600 c.p.s. is about .030, at 450,000 c.p.s. the depth of penetration is about .005"; the proper frequency for ordinary gauges of galvanized strip can be selected between these two levels.

In the following table, data is given on dull-coating practice conducted in accordance with the invention. The dull-coating practice was conducted at differing times and under varying conditions. Reference is made in this table and in the specification to the production of good tight coat. As used in the art and herein, good tight coat means that the coating will take any forming that the base metal will take Without flaking, peeling or breaking. Stated otherwise, the coating will withstand any forming that the base metal will withstand or, the coating is as ductile as the base metal. The Pittsburgh Lock Seam test, referred to in the table, is a standard test among users of galvanized product and tests coating adherence and the formability of the base metal.

Inguctiofn unit Strip speed eq. 0 opr. t rough Indicated Strip data Coating weights power input induction Heat; coloration temperature (approx.) unit, f.p.m.

Run #1"--- 28 gauge, 24" Commercial Coating, 9600 c.p.s. 25 0 to 12 Medium cherry 1,200 F. to

wide. 1.25 0z./ft. kw. to cherry. 1,400" F. Run #2 18 gauge, 36 0.8 to 1.08 oz./It 9600 c.p.s., 125 22 Dark chcrry 1,125 F.

\v e. (W. Run #3 24 gauge, 30" Commercial Coating, 0600 c.p.s., 20 to 25 Dark cherry to 1,050 F. to

wide. 1.25 0z./it. to kw. cherry. 1,250 F.

Coating Percent Percent Percent Strip thickness, in.

aluminum Al in Fe in in bath alloyed alloyed Ductility Adhesion approx. coating coating Uncoated Coated Run #1 Pittsburgh Lock Seam Good tight coat. ...i 0.21 0. 0168 0. 0137 test, very good Run #2 d d0 0. 27 0. 70 7. 69 0. 0496 0.0516 Run #3 -.d0 0. l9 49 18. 83 0. 0256 0. 0276 As brought out by practice covered in the above table a minimum strip temperature of about 1050 F. is necestary for Commercial Coatings and Light Commercial Coatings, and a temeprature between 1100 F. and 1200 F. is more practical and preferred in continuous strip operations. This is not to say that iron-zinc alloy cannot be formed at lower temperatures. Actually it is well known in the art that iron-zinc alloy layers can be formed at coating bath temperatures of 850 F. when strip is galvanized with aluminum-free spelter. However these alloy layers are microscopic and subsurface, while uniform dull-coating requires that the alloy extend to the outer surface of the coating. In practice aluminum is present in most spelter today and it retards the formation of ironzinc alloy. Why aluminum retards such alloying is not definitely known however, as brought out in the article by Hughes referred to earlier, it is believed that aluminum forms an iron-zinc-a'luminum alloy which diffuses toward the base metal. This retards the migration of iron particles into the coating to form the iron-zinc alloy. As observed by Hughes an aluminum percentage of 0.33% almost completely suppresses iron-zinc alloy at temperatures as high as 942 F. The percentage of aluminum in the galvanize coating itself must be considered in dull-coat alloying. In general, the aluminum in the coating is a higher percentage than that found in the bath. Aluminum being more reactive with iron than zinc tends to coat out, especially with heavier gauges and at slower line speeds. Therefore as the percentage of aluminum in the coating increases the proper temperature for dullcoat alloying increases. Eliminating aluminum from the bath entirely is not the solution. It must be remembered that it is necessary to extend the alloying to the outer surface of the coating in order to produce uniform dull-coat. If the aluminum is eliminated entirely the percentage of iron in the alloy, especially at the interface of the base metal and the alloy, will be increased to such an extent that coating adherence and ductility will be greatly impared. The percentage of aluminum should be chosen to maintain the iron in the alloy at between about and 20% and the temperature chosen to permit alloying to extend to the outer surface of the coating. Most continuous line operations therefore will require a temperature around 1100 F. to 1200 F. Induction heating provides the answer to raising the strip quickly to the required temperature and permitting dull-coat alloying to extend to the outer surface of the coating without diminishing line speeds to a level unacceptable to continuous strip galvanizing line operators and without damage to the coating, its adherence, or ductility.

In describing the invention various products, apparatus, and procedures have been disclosed; it is to be understood that such examples do not limit the scope of the invention and that the invention may be practiced otherwise than as specifically described Within the scope of the appended claim.

What is claimed is:

Continuous process for dull coat galvanizing of steel strip comprising applying a molten metal coating to steel by passing steel strip continuously through and out of a bath of molten zinc-aluminum alloy, then while the coating on the steel strip is still molten moving the coated steel strip into a zone of electromagnetic induction having a frequency between 9600 and 450,000 cycles per second to heat the steel by electromagnetic induction, coordinating the energy input and frequency of the electromagnetic induction and speed of the coated steel strip to heat the steel rapidly in the zone of electromagnetic induction to cause migration of iron molecules and heat from the steel into the coating to alloy iron from the steel with the zinc of the coating,

the heating of the coated steel in the zone of electromagnetic induction being at a rate suflicient to produce migration of the iron molecules to the outer surface of the coating to prevent running of the coating, and

maintaining the aluminum content of the bath between 0.10 and 0.27% and the temperature of the strip in the electromagnetic induction zone between 1050 and 1400 F. to obtain an iron content of between 10 and 20% in the coating forming an iron-zinc alloy coating of columnar crystals having a uniform matte, dull tone finish surface.

References (Iited by the Examiner UNITED STATES PATENTS 1,430,648 10/22 Herman 117--114 X 1,726,431 8/29 Fourment.

2,034,348 3/36 Lyttle.

2,197,622 4/40 Sendzimir 11757 2,345,058 3/44 Matteson 117-1 14 2,401,374 6/46 Sendzimir 117-1 14 X 2,986,808 6/61 Schnedler 29196.5 X 3,056,694 10/62 Mehler 117-1196 DAVID L. RECK, Primary Examiner. WHITMORE A. WILTZ, HYLAND BIZOT, Examiners. 

